1. Field of the Invention
The present invention relates to optical systems. More specifically, the present invention relates to systems and methods for minimizing interferometric distortion and relative intensity noise in directly modulated fiber optic links.
2. Description of the Related Art
In a fiber optic transmission system, an RF (radio frequency) signal is often modulated onto an optical carrier and delivered subsequently to a remote site for additional processing via an optical link. During the modulation and transmission process, undesirable intermodulation products can be generated, corrupting the fidelity of the transmitted signal. In addition, relative intensity noise (RIN) generated by the transmitter further reduces the spur free dynamic range (SFDR) of the system. A maximal SFDR is desirable for many applications, such as the transmission of cable TV signals.
A viable method for reducing intermodulation distortion—in particular, those originating from interferometric effects—is to employ an externally modulated link, in which the output of a CW (continuous wave) laser is externally modulated using an electro-optic (EO) modulator. Many systems currently use this approach; however, it has several undesirable properties. In particular, the manner by which the EO modulator is designed requires a relatively expensive polarization maintaining fiber for coupling the output of the CW laser to the EO modulator. Secondly, the external modulator itself is fairly expensive, resulting in an impetus to replace it with a less costly solution.
An alternative approach is to use a directly modulated diode laser as the optical source. With this approach, the CW laser and the external modulator are replaced by a single component, a directly modulated diode laser. The RF input signal is transmitted by current modulation of the diode laser to the optical fiber. This is the desirable approach to replace the more expensive externally modulated link used in present systems. In addition to being potentially lower in cost, a directly modulated link is more compact and consumes less power.
Unfortunately, the directly modulated approach suffers from corruption of the transmission signal due to interferometric distortion. To mitigate this undesirable effect, an optical isolator is typically placed between the diode laser and its fiber pigtail. The use of an optical isolator, however, increases the complexity of the optics needed to efficiently couple the emission of the diode laser to the optical fiber. Furthermore, optical isolators are typically specified to operate over limited temperature ranges. In practice, the coupling efficiencies measured—with passage through these isolators—could also vary with temperature because of differences in the coefficients of thermal expansion between the laser mount, lens/isolator mount and fiber-anchor in the transmitter package. In addition, the diode laser generates relative intensity noise, which—although small in comparison to the signal—may affect applications where very high SFDR is required.
Hence, a need exists in the art for an improved system or method for minimizing interferometric intermodulation distortion and relative intensity noise in a directly modulated fiber optic link which is lower in cost and which can maintain a high efficiency over a wider temperature range than conventional approaches.